1. Field of the Invention
The present invention relates to a method of recording an image of a desired object or symbol such as the print of a person's finger and, more particularly, to a method of providing a permanent print of such object or symbol on a recording surface containing a developing substance which reacts with an inkless reagent solution to form a colorant product representative of the object or symbol such as the ridge pattern of the person's finger.
2. Description of the Prior Art
Fingerprints have become a universal method of identifying individuals. Fingerprint identification is an exacting science since two impressions of even the same fingerprint can appear different due to variations in the amount of chemical, such as ink deposited onto the recording surface, chemical migration and changes in the finger itself. To determine an exact correspondence, a trained fingerprint technician or an automated machine reader compares the pattern of ridge endings and ridge bifurcations (minutiae) which are invariant with time on each person's fingerprint.
The prior art has recognized that any viable fingerprint identification system requires a clear distinct print pattern with a minimum of chemical migration between adjacent ridges. An additional requirement for any voluntary print identification system, such as to be utilized commercially with checks, credit verification and the like, is that it be inoffensive to the person whose fingerprint is being obtained. Ink fingerprinting systems are particularly offensive because the ink stains the finger and must be removed. Furthermore, ink systems tend to result in a smudging of the prints when the fingerprint cards, credit applications, checks, etc. are handled. Inkless fingerprinting systems have been developed to overcome the above disadvantages of ink systems.
See, for example, U.S. Pat. Nos. 4,182,261 and 4,262,623 assigned to the assignee of this application and the references cited therein. Inkless systems generally rely on a chemical reaction between an invisible reagent deposited onto a porous recording surface such as paper or a card in the form of a latent fingerprint image and a developer which is applied to the surface before or after the application of the reagent. The reagent and developer react chemically to form a colored pattern on the recording surface representative of the ridge pattern of a person's surface fingerprint. A large number of suitable reagent-developer pairs for this type of application are disclosed in U.S. Pat. Nos. 235,632, 4,029,012 and 4,182,261. It has also been suggested in U.S. Pat. No. 2,235,632 that the latent invisible fingerprint deposited from the person's finger on the recording surface maybe developed by a colorless powder, or volatile vapors comprising the developer or by the application of heat or intense light.
The application of a developer to the fingerprint paper or card just before or after the finger has deposited the reagent to provide the chemical reaction necessary to transform the invisible latent image into a visible one is time consuming and requires that an additional chemical be applied to the recording surface as compared with ink systems. The application of heat or intense light suffers from the obvious disadvantage that additional appendages to the fingerprinting apparatus, such as a heat or light source, may be required. Also, the operator of the fingerprinting apparatus cannot immediately judge whether or not the fingerprint taken is acceptable, since he or she has to wait until the print is developed by the application of the heat or light.
A need exists for an inkless fingerprint method which does not require the application of a second chemical (i.e., a developer) or heat or light to develop the latent invisible print. In a broader sense, a need exists for a method of permanently recording an image of an object or symbol on a thermosensitive recording paper or surface containing a thermosensitive color-developing layer thereon without the application of heat.
We have discovered that a substantially colorless solution containing a metallic salt when deposited in a pattern representative of an object to be reproduced, such as a fingerprint, onto a commercially available thermosensitive recording paper, as conventionally used in facsimile apparatus, will provide a permanent high contrast image of that object. We have further discovered that the application of heat is neither necessary nor desirable in such a method. Such thermosensitive recording paper includes a developer in the form of organic acid (e.g., one or more phenolic compounds) and a chromogenic substance (e.g., a colorless or pale dye such as a leuco or diazo dye). A heat sensitive barrier material generally in the form of a separation layer or protective covering encapsulating one of the reactants is normally included in such paper to prevent the chemicals from reacting absent the application of heat.
The metallic salt reacts with the organic acid (and also the dye in many cases) to provide the permanent image or print.